"Thanks to you, the commission's employees, we've taken big steps to build a future where broadband is ubiquitous and bandwidth is abundant, where innovation and investment are flourishing," he said. "Thanks to your outstanding work, America's broadband economy is thriving."

During a 22-minute speech, Genachowski read a laundry list of accomplishments by the FCC during his years as chairman. Less than a year after he took office, the FCC published a 360-page national broadband plan, which laid out a vision for faster and more available broadband across the country.

Broadband and mobile investments have risen sharply in recent years, he said. The U.S. mobile infrastructure, with as many 4G LTE subscribers in the U.S. as in all other countries combined, "is the envy of the world," said Genachowski, a technology adviser to President Barack Obama when Obama was first running for the office.

Wired broadband networks capable of 100Mbps speeds pass 80 percent of U.S. households, up from 20 percent four years ago, he added.

The FCC's accomplishments in the past four years won't be measured only by gigabits and megahertz, Genachowski said. "It'll be measured by the agency's impact on people's lives," he said. "That's why we've all come to the FCC. We're here because, if we do our job right, we can make a meaningful, positive difference in the lives of the American people."

Genachowski's FCC voted in December 2010 to pass net neutrality rules prohibiting broadband providers from selectively blocking or slowing Web traffic. Verizon Communications and MetroPCS challenged the rules in court, with the U.S. Court of Appeals for the District of Columbia Circuit expected to rule on the challenge within weeks.

During the past four years, the FCC opened up unused television spectrum, known as white spaces, to wireless broadband use and it started the process of creating incentive auctions, allowing TV stations to voluntarily give up spectrum in exchange for a portion of the auction proceeds. The auctioned TV spectrum would go to mobile broadband services, to ease an expected spectrum shortage as U.S. use of mobile broadband skyrockets.

In November 2011, Genachowski's FCC rejected a proposed acquisition of T-Mobile USA by larger competitor AT&T, with Democrats on the commission arguing the deal would significantly reduce mobile competition. This month, the commission approved a smaller merger between T-Mobile and MetroPCS.

The commission, after initially approving a plan by LightSquared to offer mobile broadband service in satellite spectrum bands, decided in February 2012 to deny the service because of interference concerns to GPS devices operating in nearby bands.

The commission also overhauled the federal Universal Service Fund, a program that provides subsidies for telephone service and refocused the fund on broadband.

Genachowski's decision follows an announcement this week from Robert McDowell, the longest-serving member of the FCC, that he would leave the commission in the coming weeks. With the departures of Genachowski and McDowell, two of the three remaining members of the commission have served less than a year.

While Genachowski laid out several accomplishments, some consumer and digital rights groups said he didn't do enough to protect consumers and promote broadband competition. Genachowski's FCC has allowed large broadband and mobile providers to grow while doing little for smaller competitors, his critics said.

Genachowski's appointment raised "high hopes" that he would promote the public interest, Free Press President and CEO Craig Aaron said in a statement. "But instead of acting as the people's champion, he's catered to corporate interests," Aaron said. "His tenure has been marked by wavering and caving rather than the strong leadership so needed at this crucial agency."

Genechowski's net neutrality rules are "full of loopholes and offer no guarantee that the FCC will be able to protect consumers from corporate abuse in the future," Aaron said.

While the FCC took several positive steps during the past four years, Genachowski's term "can best be described as one of missed opportunities," digital rights group Public Knowledge said in a statement. "He had the opportunity, but declined, to solidify the agency's authority and ability to protect consumers with regard to broadband -- the communications system of the present and future."

With the agency under Genachowski declining to assert regulatory authority over broadband, the FCC could become "a powerless and irrelevant agency" as U.S. networks transition to an all-Internet Protocol core, the group said.

Other groups were more complimentary of Genachowski. He will be seen a the "spectrum chairman" who focused on expanding mobile spectrum for broadband, Consumer Electronics Association President and CEO Gary Shapiro said in a statement. CEA called Genachowski "a visionary leader, an adept regulator, and a true friend to the innovation society."

Genechowski had a strong focus on broadband, added Jim Cicconi, AT&T's vice president of external and legislative affairs. The chairman identified a spectrum crisis and helped push for legislation that will lead to the incentive spectrum auctions, Cicconi said in a statement. Genachowski's accomplishments "are going to leave a lasting imprint on communications policy in the United States," he said.

Grant Gross covers technology and telecom policy in the U.S. government for The IDG News Service. Follow Grant on Twitter at GrantGross. Grant's e-mail address is grant_gross@idg.com.